Intervertebral spacer device utilizing a belleville washer having radially extending grooves

ABSTRACT

An intervertebral spacer device having a pair of opposing plates for seating against opposing vertebral bone surfaces, coupled to one another by a convex element mounted to one of the plates and maintained against the other plate by a retaining wall and a retaining ring.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 09/968,046 filed Oct. 1, 2001 now abandonedentitled “Intervertebral Spacer Device Utilizing a Belleville WasherHaving Radially Extending Grooves”, abandoned, which is fullyincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to a spinal implant assembly forimplantation into the intervertebral space between adjacent vertebralbones to simultaneously provide stabilization and continued flexibilityand proper anatomical motion, and more specifically to such a devicewhich utilizes a belleville washer, having radially extending grooves,as a restoring force generating element.

BACKGROUND OF THE INVENTION

The bones and connective tissue of an adult human spinal column consistsof more than 20 discrete bones coupled sequentially to one another by atri-joint complex which consists of an anterior disc and the twoposterior facet joints, the anterior discs of adjacent bones beingcushioned by cartilage spacers referred to as intervertebral discs.These more than 20 bones are anatomically categorized as being membersof one of four classifications: cervical, thoracic, lumbar, or sacral.The cervical portion of the spine, which comprises the top of the spine,up to the base of the skull, includes the first 7 vertebrae. Theintermediate 12 bones are the thoracic vertebrae, and connect to thelower spine comprising the 5 lumbar vertebrae. The base of the spine isthe sacral bones (including the coccyx). The component bones of thecervical spine are generally smaller than those of the thoracic spine,which are in turn smaller than those of the lumbar region. The sacralregion connects laterally to the pelvis. While the sacral region is anintegral part of the spine, for the purposes of fusion surgeries and forthis disclosure, the word spine shall refer only to the cervical,thoracic, and lumbar regions.

The spinal column of bones is highly complex in that it includes overtwenty bones coupled to one another, housing and protecting criticalelements of the nervous system having innumerable peripheral nerves andcirculatory bodies in close proximity. In spite of these complications,the spine is a highly flexible structure, capable of a high degree ofcurvature and twist in nearly every direction.

Genetic or developmental irregularities, trauma, chronic stress, tumors,and degenerative wear are a few of the causes that can result in spinalpathologies for which surgical intervention may be necessary. A varietyof systems have been disclosed in the art which achieve immobilizationand/or fusion of adjacent bones by implanting artificial assemblies inor on the spinal column. The region of the back which needs to beimmobilized, as well as the individual variations in anatomy, determinethe appropriate surgical protocol and implantation assembly. Withrespect to the failure of the intervertebral disc, the interbody fusioncage has generated substantial interest because it can be implantedlaparoscopically into the anterior of the spine, thus reducing operatingroom time, patient recovery time, and scarification.

Referring now to FIGS. 1 and 2, in which a side perspective view of anintervertebral body cage and an anterior perspective view of a postimplantation spinal column are shown, respectively, a more completedescription of these devices of the prior art is herein provided. Thesecages 10 generally comprise tubular metal body 12 having an externalsurface threading 14. They are inserted transverse to the axis of thespine 16, into preformed cylindrical holes at the junction of adjacentvertebral bodies (in FIG. 2 the pair of cages 10 are inserted betweenthe fifth lumbar vertebra (L5) and the top of the sacrum (S1). Two cages10 are generally inserted side by side with the external threading 14tapping into the lower surface of the vertebral bone above (L5), and theupper surface of the vertebral bone (S1) below. The cages 10 includeholes 18 through which the adjacent bones are to grow. Additionalmaterial, for example autogenous bone graft materials, may be insertedinto the hollow interior 20 of the cage 10 to incite or accelerate thegrowth of the bone into the cage. End caps (not shown) are oftenutilized to hold the bone graft material within the cage 10.

These cages of the prior art have enjoyed medical success in promotingfusion and grossly approximating proper disc height. It is, however,important to note that the fusion of the adjacent bones is an incompletesolution to the underlying pathology as it does not cure the ailment,but rather simply masks the pathology under a stabilizing bridge ofbone. This bone fusion limits the overall flexibility of the spinalcolumn and artificially constrains the normal motion of the patient.This constraint can cause collateral injury to the patient's spine asadditional stresses of motion, normally borne by the now-fused joint,are transferred onto the nearby facet joints and intervertebral discs.It would therefore, be a considerable advance in the art to provide animplant assembly which does not promote fusion, but, rather, whichnearly completely mimics the biomechanical action of the natural disccartilage, thereby permitting continued normal motion and stressdistribution.

It is, therefore, an object of the present invention to provide a newand novel intervertebral spacer which stabilizes the spine withoutpromoting a bone fusion across the intervertebral space.

It is further an object of the present invention to provide an implantdevice which stabilizes the spine while still permitting normal motion.

It is further an object of the present invention to provide a device forimplantation into the intervertebral space which does not promote theabnormal distribution of biomechanical stresses on the patient's spine.

Other objects of the present invention not explicitly stated will be setforth and will be more clearly understood in conjunction with thedescriptions of the preferred embodiments disclosed hereafter.

SUMMARY OF THE INVENTION

The preceding objects of the invention are achieved by the presentinvention which is a flexible intervertebral spacer device comprising apair of spaced apart base plates, arranged in a substantially parallelplanar alignment (or slightly offset relative to one another inaccordance with proper lordotic angulation) and coupled to one anotherby means of a spring mechanism. In particular, this spring mechanismprovides a strong restoring force when a compressive load is applied tothe plates, and may also permit rotation of the two plates relative toone another. While there are a wide variety of embodiments contemplated,a preferred embodiment includes a belleville washer utilized as therestoring force providing element, the belleville washer having radiallyextending grooves.

More particularly, as the assembly is to be positioned between thefacing surfaces of adjacent vertebral bodies, the base plates shouldhave substantially flat external surfaces which seat against theopposing bone surfaces. Inasmuch as these bone surfaces are oftenconcave, it is anticipated that the opposing plates may be convex inaccordance with the average topology of the spinal anatomy. In addition,the plates are to mate with the bone surfaces in such a way as to notrotate relative thereto. (The plates rotate relative to one another, butnot with respect to the bone surfaces to which they are each in contactwith.) In order to prevent rotation of a plate relative to the bone, theupper and lower plates can include a porous coating into which the boneof the vertebral body can grow. (Note that this limited fusion of thebone to the base plate does not extend across the intervertebral space.)

In some embodiments (not in the preferred embodiment), between the baseplates, on the exterior of the device, there is included acircumferential wall which is resilient and which simply preventsvessels and tissues from entering within the interior of the device.This resilient wall may comprise a porous fabric or a semi-impermeableelastomeric material. Suitable tissue compatible materials meeting thesimple mechanical requirements of flexibility and durability areprevalent in a number of medical fields including cardiovascularmedicine, wherein such materials are utilized for venous and arterialwall repair, or for use with artificial valve replacements.Alternatively, suitable plastic materials are utilized in the surgicalrepair of gross damage to muscles and organs. Still further materialsthat could be utilized herein may be found in the field of orthopedic inconjunction with ligament and tendon repair. It is anticipated thatfuture developments in this area will produce materials that arecompatible for use with this invention, the breadth of which shall notbe limited by the choice of such a material.

As introduced above, the internal structure of the present inventioncomprises a spring member, which provides a restoring force whencompressed. More particularly, it is desirable that the restoring forcesbe directed outward against the opposing plates, when a compressive loadis applied to the plates. In addition, in certain embodiments, it isnecessary that the restoring force providing subassembly notsubstantially interfere with the rotation of the opposing platesrelative to one another. In the preferred embodiment, the springsubassembly is configured to allow rotation of the plates relative toone another. In other embodiments, the spring subassembly can beconfigured to either allow rotation of the plates, or prevent rotationof the plates (through the tightening of a set screw as discussedbelow). As further mentioned above, the force restoring member comprisesat least one belleville washer.

Belleville washers are washers which are generally bowed in the radialdirection. Specifically, they have a radial convexity (i.e., the heightof the washer is not linearly related to the radial distance, but may,for example, be parabolic in shape). The restoring force of a bellevillewasher is proportional to the elastic properties of the material. Inaddition, the magnitude of the compressive load support and therestoring force provided by the Belleville washer may be modified byproviding grooves in the washer. In the present invention, thebelleville washer utilized as the force restoring member has radiallyextending grooves that decrease in width and depth from the outside edgeof the washer toward the center of the washer.

As a compressive load is applied to a bellevilie washer, the forces aredirected into a hoop stress which tends to radially expand the washer.This hoop stress is counterbalanced by the material strength of thewasher, and the strain of the material causes a deflection in the heightof the washer. Stated equivalently, a belleville washer responds to acompressive load by deflecting compressively, but provides a restoringforce which is proportional to the elastic modulus of the material in ahoop stressed condition. With grooves formed in the washer, it expandsand restores itself far more elastically than a solid washer.

In general, the belleville washer is one of the strongest configurationsfor a spring, and is highly suitable for use as a restoring forceproviding subassembly for use in an intervertebral spacer element whichmust endure considerable cyclical loading in an active human adult.

In the preferred embodiment of the present invention, a single modifiedbelleville washer, which has radially extending grooves as describedabove, is utilized in conjunction with a ball-shaped post on which it isfree to rotate through a range of angles (thus permitting the plates torotate relative to one another through a corresponding range of angles).More particularly, this embodiment comprises a pair of spaced apart baseplates, one of which is simply a disc shaped member (preferably shapedto match the end of an intervertebral disc) having an external face(having the porous coating discussed above) and an internal face havingan annual retaining wall (the purpose of which will be discussed below).The other of the plates is similarly shaped, having an exterior facewith a porous coating, but further includes on its internal face acentral post portion which rises out of the internal face at a nearlyperpendicular angle. The top of this post portion includes a ball-shapedknob. The knob includes a central threaded axial bore which receives asmall set screw. Prior to the insertion of the set screw, theball-shaped head of the post can deflect radially inward (so that theball-shaped knob contracts). The insertion of the set screw eliminatesthe capacity for this deflection.

As introduced above, a modified belleville washer having radiallyextending grooves is mounted to this ball-shaped knob in such a way thatit may rotate freely through a range of angles equivalent to thefraction of normal human spine rotation (to mimic normal disc rotation).The belleville washer of this design is modified by including anenlarged inner circumferential portion (at the center of the washer)which accommodates the ball-shaped portion of the post. Moreparticularly, the enlarged portion of the modified belleville washerincludes a curvate volume having a substantially constant radius ofcurvature which is also substantially equivalent to the radius of theball-shaped head of the post. The deflectability of the ball-shaped headof the post, prior to the insertion of the set screw, permits the headto be inserted into the interior volume at the center of the bellevillewasher. Subsequent introduction of the set screw into the axial bore ofthe post prevents the ball-shaped head from deflecting. Thereby, thewasher can be secured to the ball-shaped head so that it can rotatethereon through a range of proper lordotic angles (in some embodiments,a tightening of the set screw locks the washer on the ball-shaped headat one of the lordotic angles).

This assembly provides ample spring-like performance with respect toaxial compressive loads, as well as long cycle life to mimic the axialbiomechanical performance of the normal human intervertebral disc. Theradially extending grooves of the belleville washer allow the washer toexpand radially as the grooves widen under the load, only to spring backinto its undeflected shape upon the unloading of the spring. As thewasher compresses and decompresses, the annular retaining wall maintainsthe wide end of the washer within a prescribed boundary on the internalface of the base plate which it contacts, and an annular retaining ringmaintains the wide end of the washer against the internal face.

Finally, inasmuch as the human body has a tendency to produce fibroustissues in perceived voids, such as may be found within the interior ofthe present invention, and such fibrous tissues may interfere with thestable and/or predicted functioning of the device, some embodiments ofthe present invention (although not the preferred embodiment) will befilled with a highly resilient elastomeric material. The material itselfshould be highly biologically inert, and should not substantiallyinterfere with the restoring forces provided by the spring-likemechanisms therein. Suitable materials may include hydrophilic monomerssuch as are used in contact lenses. Alternative materials includesilicone jellies and collagens such as have been used in cosmeticapplications. As with the exterior circumferential wall, which wasdescribed above as having a variety of suitable alternative materials,it is anticipated that future research will produce alternatives to thematerials described herein, and that the future existence of suchmaterials which may be used in conjunction with the present inventionshall not limit the breadth thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side perspective view of an interbody fusion device of theprior art.

FIG. 2 is a front view of the anterior portion of the lumbo-sacralregion of a human spine, into which a pair of interbody fusion devicesof the type shown in FIG. 1 have been implanted.

FIGS. 3 a and 3 b are side cross-section views of the upper and loweropposing plates of the preferred embodiment of the present invention.

FIGS. 4 a and 4 b are top and side cross-section views of a bellevillewasher having radially extending grooves, for use in a preferredembodiment of the present invention.

FIG. 5 a is a top view of the upper plate of FIG. 3 a, with thebelleville washer of FIGS. 4 a and 4 b fitted within a retaining walland a retaining ring of the upper plate.

FIG. 5 b is a top view of the lower plate of FIG. 3 b.

FIG. 6 is a side cross-section view of the preferred embodiment of thepresent invention, which utilizes a Belleville washer of the type shownin FIGS. 4 a and 4 b, showing the plates of FIGS. 5 a and 5 b assembledtogether.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While the present invention will be described more fully hereinafterwith reference to the accompanying drawings, in which particularembodiments and methods of implantation are shown, it is to beunderstood at the outset that persons skilled in the art may modify theinvention herein described while achieving the functions and results ofthis invention. Accordingly, the descriptions which follow are to beunderstood as illustrative and exemplary of specific structures, aspectsand features within the broad scope of the present invention and not aslimiting of such broad scope. Like numbers refer to similar features oflike elements throughout.

Referring now to FIGS. 3 a and 3 b, side cross-section views of upperand lower plate members 100,200 of the preferred embodiment of thepresent invention are shown. As the device is designed to be positionedbetween the facing surfaces of adjacent vertebral bodies, the platesinclude substantially flat external face portions 102,202 which seatagainst the opposing bone surfaces. In addition, the plates are to matewith the bone surfaces in such a way as to not rotate relative thereto.It is, therefore, preferred that the external faces of the platesinclude a porous coating 104,204 into which the bone of the vertebralbody can grow. (Note that this limited fusion of the bone to the baseplate does not extend across the intervertebral space.) A hole (notshown) can be provided in the upper plate such that the interior of thedevice may be readily accessed if a need should arise.

The upper plate 100 includes an internal face 103 that includes anannular retaining wall 108 and an annular retaining ring 109. The lowerplate 200 includes an internal face 203 that includes a central postmember 201 which rises out of the internal face 203 at a nearlyperpendicular angle. The top of this post member 201 includes aball-shaped head 207. The head 207 includes a series of slots whichrender it compressible and expandable in correspondence with a radialpressure (or a radial component of a pressure applied thereto). The head207 includes a central threaded axial bore 209 which extends down thepost 201. This threaded bore 209 is designed to receive a set screw 205.Prior to the insertion of the set screw 205, the ball-shaped head 207 ofthe post 201 can deflect radially inward because of the slots (so thatthe ball-shaped head contracts). The insertion of the set screw 205eliminates the capacity for this deflection.

Referring now to FIGS. 4 a and 4 b, a belleville washer 130 havingradially extending grooves is provided in top and side cross-sectionviews. The belleville washer 130 is a restoring force providing devicewhich comprises a circular shape, having a central opening 132, andwhich is radially arched in shape. The belleville washer 130 has aradial convexity 134 (i.e., the height of the washer 130 is not linearlyrelated to the radial distance, but may, for example, be parabolic inshape). The restoring force of the belleville washer 130 is proportionalto the elastic properties of the material.

The belleville washer 130 comprises a series of grooves 133 formedtherein. The grooves 133 extend radially from the outer edge of thebelleville washer toward the center of the element. In the preferredembodiment, the width and depth of each groove 133 decreases along thelength of the groove 133 from the outer edge of the washer toward thecenter of the washer, such that the center of the washer is flat, whilethe outer edge of the washer has grooves of a maximum groove depth. Itshould be understood that in other embodiments, one or both of the depthand the width of each groove can be (1) increasing along the length ofthe groove from the outer edge of the washer toward the center of thewasher, (2) uniform along the length of the groove from the outer edgeof the washer toward the center of the washer, or (3) varied along thelength of each groove from the outer edge of the washer toward thecenter of the washer, either randomly or according to a pattern.Moreover, in other embodiments, it can be the case that each groove isnot formed similarly to one or more other grooves, but rather one ormore grooves are formed in any of the above-mentioned fashions, whileone or more other grooves are formed in another of the above-mentionedfashions or other fashions. It should be clear that any groove patterncan be implemented without departing from the scope of the presentinvention.

As a compressive load is applied to the belleville washer 130, theforces are directed into a hoop stress which tends to radially expandthe washer. This hoop stress is counterbalanced by the material strengthof the washer, and the force necessary to widen the radial grooves 133along with the strain of the material causes a deflection in the heightof the washer. Stated equivalently, the belleville washer 130 respondsto a compressive load by deflecting compressively; the radial groovescause the washer to further respond to the load by spreading as thegrooves in the washer expand under the load. The spring, therefore,provides a restoring force which is proportional to the elastic modulusof the material in a hoop stressed condition.

More particularly, the central opening 132 of the belleville washer isenlarged. This central opening 132 includes a curvate volume 233 forreceiving therein the ball-shaped head 207 of the post 201 of the lowerplate 200 described above. More particularly, the curvate volume 233 hasa substantially constant radius of curvature which is also substantiallyequivalent to the radius of the ball-shaped head 207 of the post 201. Inthis embodiment, the depth of each groove 133 decreases along the lengthof the groove 133 from the outer edge of the washer toward the center ofthe washer, such that the center of the washer is flat, while the outeredge of the washer has grooves of a maximum groove depth. Therefore, thecentral opening 132 can be formed from flat edges. It should beunderstood that this is not required, but rather is preferred for thisembodiment.

Referring now to FIG. 5 a, a top view of the upper plate 100 of FIG. 3a, with the radially grooved belleville washer 130 of FIGS. 4 a and 4 bfitted within a retaining wall 108 and a retaining ring 109 of the upperplate 100, is shown. The diameter of the retaining wall 108 ispreferably slightly wider than the diameter of the undeflectedbelleville washer 130 such that the loading thereof can result in anunrestrained radial deflection of the washer 130. FIG. 5 b shows a topview of the lower plate 200 of FIG. 3 b.

Referring also to FIG. 6, which shows the fully assembled preferredembodiment of the present invention is shown. The radially groovedbelleville washer 130 is placed with its wide end against the top plate100 within the annular retaining wall 108 as shown in FIG. 6. Theannular retaining ring 109 is provided to hold the belleville washer 130against the internal face 103 of the upper plate 100 within theretaining wall 108. The post 201 of the lower plate 200 is fitted intothe central opening 132 of the belleville washer 130 (the deflectabilityof the ball-shaped head 207 of the post 201, prior to the insertion ofthe set screw 205, permits the head 207 to be inserted into the interiorvolume 233 at the center of the belleville washer 130. Subsequentintroduction of the set screw 205 into the axial bore 209 of the post201 eliminates the deflectability of the head 207 so that the washer 130cannot be readily removed therefrom, but can still rotate thereon. Insome embodiments (not in this preferred embodiment), the post head 207can be locked tightly within the central volume 233 of the bellevillewasher 130 by the tightening of the set screw 205, to prevent anyrotation of the plates 100,200. Compressive loading of the assemblycauses the washer 130 to deflect (with the radially extending groovesenhancing the deflection) so that the wide end radially expands whilebeing maintained centrally against the upper plate 100 by the retainingwall 108 and the retaining ring 109. When the load is removed, thewasher 130 springs back to its original shape.

Inasmuch as the human body has a tendency to produce fibrous tissues inperceived voids, such as may be found within the interior of the presentinvention, and such fibrous tissues may interfere with the stable and/orpredicted functioning of the device, some embodiments of the presentinvention (although not the preferred embodiment) will be filled with ahighly resilient and biologically inert elastomeric material. Suitablematerials may include hydrophilic monomers such as are used in contactlenses. Alternative materials include silicone jellies and collagenssuch as have been used in cosmetic applications.

While there has been described and illustrated embodiments of anintervertebral spacer device, it will be apparent to those skilled inthe art that variations and modifications are possible without deviatingfrom the broad spirit and principle of the present invention. Theinvention, therefore, shall not be limited to the specific embodimentsdiscussed herein.

1. An intervertebral spacer device comprising: first and second plate members, each having first and second plate surfaces thereof, said plate members being disposed in a spaced apart relationship such that said first plate surfaces oppose one another, and said second plate surfaces face in opposite directions; and a convex element by which said first and second plate members are coupled to one another, by said convex element being mounted to said second plate member and maintained against said first plate member, and being disposed such that at least a portion of a load applied to said second plate surfaces is transmitted to said convex element, said mounting of said convex element to said second plate member establishing a center of rotation between said first and second plate surfaces about which said first and second plate members rotate relative to one another, said mounting of said convex element to said second plate member and said maintenance of said convex element against said first plate member together preventing said first and second plate members from becoming uncoupled from one another under a load applied to said first and second plate surfaces; wherein said convex element is maintained against said first plate member by a retaining wall and a retaining ring, the retaining wall being on the first plate member and circumferentially surrounding a wide end of said convex element, the retaining ring extending from the retaining wall such that at least a portion of the wide end of said convex element is disposed between said first plate member and the retaining ring, such that said convex element is held against said first plate member by the retaining ring and the retaining wall, and wherein the retaining wall and the retaining ring define a substantially enclosed area over said first plate surface of said first plate member and wherein upon implantation of said assembled spacer device said convex element is movable relative to said first plate member within the enclosed area.
 2. The device as set forth in claim 1, wherein said convex element is mounted to said second plate member by a ball captured in a curvate volume provided by said convex element.
 3. The device as set forth in claim 2, wherein said convex element includes a belleville washer having the wide end and a narrow end having the curvate volume.
 4. The device as set forth in claim 1, wherein said mounting of said convex element to said second plate member includes a post structure extending outwardly from said second plate member, which post structure includes said ball at an end of said post structure.
 5. The device as set forth in claim 4, wherein said mounting of said convex element to said second plate member further includes a threaded bore in said post structure that extends axially from said ball head toward said second plate member, and which bore receives therein a threaded set screw such that prior to an insertion of the set screw therein, said bore permits the ball to compress radially inwardly, and such that after the insertion of said set screw said ball is not readily radially compressible.
 6. The device as set forth in claim 4, wherein said mounting of said convex element to said second plate member further includes said curvate volume, said convex element includes a belleville washer, and said belleville washer includes a central opening that includes said curvate volume for receiving and holding therein said ball.
 7. The device as set forth in claim 1, wherein the retaining wall is in contact with and extends away from the first plate surface of said first plate.
 8. The device as set forth in claim 1, wherein the retaining wall is in contact with the first plate surface of said first plate.
 9. The device as set forth in claim 1, wherein the retaining wall has a first diameter and said first plate has an outer perimeter defining a second diameter that is larger than the first diameter.
 10. The device as set forth in claim 1, wherein the retaining wall and the retaining ring cooperate for limiting movement of said convex element over the first plate surface of said first plate.
 11. The device is claimed in claim 1, wherein the retaining wall is stationary relative to said first plate member and said convex element is movable relative to said first plate member and said retaining wall. 